Thermochemistry

Thermochemistry is a branch of chemical thermodynamics whose task is to determine and study the thermal effects of reactions , as well as to establish their relationships with various physicochemical parameters. Another of the tasks of thermochemistry is the measurement of the heat capacities of substances and the establishment of their heats of phase transitions .

Content

1 Basic concepts and laws of thermochemistry
- 1.1 Thermochemical equations
- 1.2 Hess's Law
- 1.3 Kirchhoff's Law
- 1.4 Methods of thermochemistry
2 See also
3 Literature

Basic concepts and laws of thermochemistry

Thermochemical equations

Thermochemical equations of reactions are equations in which the aggregate states of these compounds or crystallographic modification are indicated near the symbols of chemical compounds and the numerical values of thermal effects are indicated on the right side of the equation

The most important quantity in thermochemistry is the standard heat of formation ( standard enthalpy of formation ). The standard heat (enthalpy) of formation of a complex substance is the thermal effect (change in standard enthalpy) of the reaction of formation of one mole of this substance from simple substances in a standard state. The standard enthalpy of formation of simple substances in this case is taken equal to zero.

In the thermochemical equations it is necessary to indicate the state of aggregation of substances using letter indices, and the thermal effect of the reaction (ΔН) should be written separately, separated by commas. For example, the thermochemical equation

4NH ₃ (g) + 3O ₂ (g) → 2N ₂ (g) + 6H ₂ O (g), ΔН = -1531 kJ

shows that this chemical reaction is accompanied by the release of 1531 kJ of heat, at a pressure of 101 kPa , and refers to the number of moles of each of the substances that corresponds to the stoichiometric coefficient in the reaction equation. In thermochemistry, equations are also used in which the thermal effect is attributed to one mole of the formed substance, using fractional coefficients if necessary.

The thermal effect of a chemical reaction is equal to the difference between the total enthalpy of formation of all reaction products and all starting materials, taking into account stoichiometric coefficients (the number of moles of reacted substances). That is, the thermal effect of a chemical reaction is calculated by the general expression:

ΔH = (∑ΔH _products ) - (∑ΔH _source )

Thus, the more stable the reaction products and the higher the internal energy of the starting compounds, the higher the thermal effect of the reaction, which is a direct consequence of the law of minimum energy and maximum entropy . To calculate the thermal effects of reactions under standard conditions, standard enthalpies of compound formation are taken from reference tables.

Hess's Law

The basis of thermochemical calculations is the Hess law: The thermal effect (∆Н) of a chemical reaction (at constant P and T) depends on the nature and physical state of the starting materials and reaction products and does not depend on its path.

Consequences from the law of Hess:

The thermal effects of the direct and reverse reactions are equal in magnitude and opposite in sign.
The thermal effect of the chemical reaction (∆Н) is equal to the difference between the sum of the enthalpies of formation of the reaction products and the sum of the enthalpies of the formation of the starting materials, taking into account the coefficients in the reaction equation (that is, multiplied by them).

Hess's law can be written as the following mathematical expression:

\Delta H^{\theta }=\Sigma (\Delta H_{f~products}^{\theta })-\Sigma (\Delta H_{f~reactants}^{\theta })

{\ displaystyle \ Delta H ^ {\ theta} = \ Sigma (\ Delta H_ {f ~ products} ^ {\ theta}) - \ Sigma (\ Delta H_ {f ~ reactants} ^ {\ theta})}

.

Using the Hess law, one can calculate the enthalpies of the formation of substances and the thermal effects of reactions that cannot be measured experimentally.

Kirchhoff Law

Kirchhoff's law establishes the dependence of the thermal effect of a chemical reaction on temperature: the temperature coefficient of the thermal effect of a chemical reaction is equal to the change in the heat capacity of the system during the reaction. Kirchhoff's law underlies the calculation of thermal effects at different temperatures.